As coating methods of car bodies, generally those for forming multilayer coating film by 3-coat-2-bake (3C2B) system comprising, after applying an electrodeposition paint on the coating object, application of intermediate paint→curing by baking→application of water-based base coating paint→preheating (preliminary heating)→application of clear coating paint→curing by baking, have been widely adopted. Whereas, for energy-saving, attempts are made in recent years to omit the bake-curing step after application of intermediate paint and adopt 3-coat-1-bake (3C1B) system comprising, after applying an electrodeposition paint on the coating object, application of intermediate paint→preheating (preliminary hearing)→application of water-based base coating paint→preheating (preliminary heating)→application of clear coating paint→curing by baking (e.g., see Patent Document 1).
However, because the intermediate paint and water-based base coating paint are applied one on another in uncured condition in the above 3-coat-1-bake system, layer mixing is apt to take place between adjacent coating films, which occasionally impairs smoothness or distinctness of image of resulting coating film.
As a countermeasure thereto, a method for suppressing such layer mixing of the intermediate coat with the water-based base coat thereon was tried in the past, wherein a curing agent which reacts at relatively low temperatures such as a polycarbodiimide compound was blended in the intermediate paint. This method, however, could not form a coating film having sufficient smoothness.
For example, Patent Document 2 disclosed that multilayer coating films having a high quality appearance could be obtained while controlling interfacial bleeding or inversion between coating film layers, in aforesaid 3-coat-1-bake coating system, when the water-based intermediate paint and/or water-based metallic base paint contains a polycarbodiimide compound and a carboxyl-containing aqueous resin. However, multilayer coating films formed by said method occasionally have insufficient smoothness.